Liposomes are unilamellar or multilamellar lipid vesicles which enclose a fluid space. The walls of the vesicles are formed by a bimolecular layer of one or more lipid components having polar heads and non-polar tails. In an aqueous (or polar) solution, the polar heads of one layer orient outwardly to extend into the surrounding medium, and the non-polar tail portions of the lipids associate with each other, thus providing a polar surface and a non-polar core in the wall of the vesicle. Unilamellar liposomes have one such bimolecular layer, whereas multilamellar liposomes generally have a plurality of substantially concentric bimolecular layers.
A variety of methods for preparing liposomes are known, many of which have been described by Szoka and Papahadjopoulos, Ann. Rev. Biophysics Bioeng. 9:467-508 (1980). Also, several liposome encapsulation methods are disclosed in the patent literature, notably in U.S. Pat. Nos. 4,235,871, to Papahadjopoulos et al., issued Nov. 25, 1980, and in 4,016,100 to Suzuki et al., issued Apr. 5, 1977.
Liposomes are well recognized as useful for encapsulation of drugs and other therapeutic agents and for carrying these agents to in vivo sites. For example, U.S. Pat. No. 3,993,754, inventors Rahman et al., issued Nov. 23, 1976, discloses an improved chemotherapy method in which an anti-tumor drug is encapsulated within liposomes and then injected. U.S. Pat. No. 4,263,428, inventors Apple et al., issued Apr. 21, 1981, discloses an antitumor drug which may be more effectively delivered to selective cell sites in a mammalian organism by incorporating the drug within uniformly sized liposomes. Drug administration via liposomes can have reduced toxicity, altered tissue distribution, increased drug effectiveness, and an improved therapeutic index.
Although encapsulation of therapeutic agents and biologically active compounds in liposomes has significant commercial potential, a major difficulty that has been encountered in the commercial use of liposome encapsulates is with their long term stability.
Attempts have been made to preserve liposomes (and their encapsulated contents) by freeze-drying with various preserving agents, but the lyophilizates have lost a significant part of the encapsulated contents during the lyophilization and rehydration. U.S. Pat. No. 4,411,894, inventors Shrank et al., issued Oct. 25, 1983, discloses sonicated liposomes encapsulating fat-soluble pharmaceuticals which have a high sucrose concentration (at least 0.4 molar), and then the composition may be lyophilized. However, even at the pharmaceutically enormous amounts of sucrose disclosed by the Shrank et al. patent, there is still a significant loss of encapsulated contents (about 30%).
Crommelin et al., Pharmaceutical Research, pages 159-163 (1984) investigated the storage stability of liposomes when freeze-dried, frozen or as an aqueous dispersion. Crommelin et al. added lactose externally to liposomes, then froze and thawed the vesicles, but found the vesicles lost between 35 and 60% of the encapsulated contents. This loss was not decreased by the external addition of lactose.